For the purpose of forming a connection to a main pipeline so that a branch pipeline of smaller diameter is joined thereto, it has been known to put onto the main pipeline molded pieces, called molded centering pieces, and to produce the connection with the branch line connected to the molded piece by spot drilling of the main line. Such centering molded pieces are, in most instances, formed in two parts which are somewhat saddle-shaped, the two saddle halves being held together, for example, by wedges as shown in Swiss Pat. No. 464,260, or the two halves are held together, in the case of plastic lines, by a welded connection with the main pipeline as shown in Swiss Pat. No. 528,697.
In all known molded centering pieces, the connecting sleeve for the branch line is provided at the centering sleeve for the hole-forming tool, extending radially to the main pipeline with the connecting piece. Such an arrangement only permits the connection of branch lines which have a cross-section or diameter considerably smaller than that of the main pipeline if the connection structure is to be a compact arrangement.
Branch lines with a diameter at least as large, or only slightly smaller, than the main pipeline may, to be sure, be produced with commercially available molded T-shaped pieces, but this is expensive and it is not possible to use this kind of structure when the branch connection is to be made while maintaining the operating pressure of the medium being conducted through the main pipeline.